1. Field of the Invention
The present invention relates to a drift tube linear accelerator for accelerating charged particles, such as protons or heavy particles.
2. Description of the Background Art
In order to accelerate charged particles, such as protons or heavy particles to high energy, a synchrotron is utilized. In the synchrotron, an injector for pre-acceleration is used. Typically, the injector is configured with an ion source, a pre-accelerator and a post-accelerator. As the post-accelerator, a drift tube linear accelerator is applied.
The drift tube linear accelerator is configured with an acceleration cavity in which several or several tens of electrodes called as drift tubes are arranged in one direction of an acceleration-beam axis. The acceleration cavity is a resonator having a resonance frequency. When high-frequency power corresponding to the resonance frequency of the acceleration cavity is supplied to the acceleration cavity, a high-frequency electric field is generated between the drift tube electrodes. Charged particles such as protons entered into the acceleration cavity are accelerated by receiving energy from the high-frequency electric field generated between the drift tube electrodes. When, due to time-wise (phase) variation of the high-frequency electric field, the electric field is generated in reverse direction against the accelerating direction, the charged particles are decelerated. Thus, the arrangement of the drift tube electrodes is so designed that the charged particles are to be accelerated. That is, the arrangement of the drift tube electrodes is designed such that the charged particles stay in between the drift tube electrodes when an accelerating electric field is generated, whereas the charges particles stay in the drift tube electrodes when a decelerating electric field is generated, so as to avoid adverse effect by the generated electric field.
Examples in structure of the drift tube linear accelerator include an Alvarez-type linear accelerator and an IH (Interdigital-H)-type linear accelerator. The Alvarez-type linear accelerator is characterized by its 2π-mode acceleration in which the phase goes by 360 degree from a center between drift tube electrodes to next center between drift tube electrodes. Thus, the drift tube electrodes have a sufficient length to allow divergence of the charged particles. Therefore, in order to prevent the divergence of the charged particles, a focusing device such as quadrupole electrode, etc., for suppressing the divergence of the charged particles is generally disposed in the drift tube electrode. Consequently, as an injector for accelerating charged particles that are light in mass and to be easily diverged, such as protons, the Alvarez-type accelerator that allows the addition of quadrupole electrode, etc., is adopted.
In contrast, the IH-type linear accelerator is characterized by its n-mode acceleration in which the phase goes by 180 degree from a center between drift tube electrodes to next center between drift tube electrodes. Thus, the IH-type linear accelerator achieves an acceleration frequency that is made twice that of the Alvarez-type linear accelerator, so that the whole length of the drift tube electrode can be shorter than that of the Alvarez-type linear accelerator; however, when the whole length is short, it is difficult to dispose the focusing device such as quadrupole electrode, etc., in the drift tube electrode in order to prevent the divergence of the charged particles. Consequently, as an injector for accelerating charged particles that are heavy in mass and not to be easily diverged, such as carbon ions, the IH-type accelerator is adopted also because the whole length can be short.
The injector is a device for preliminarily accelerating the particles to the energy receivable by the synchrotron, and thus it is necessary to satisfy the requirements by the synchrotron for reception. In particular, not only the energy but also its difference between the charged particles (referred to as “momentum spread”) is required to fall within a specified range. In this instance, in order to achieve a planned accelerating electric-field distribution, the drift tube linear accelerator is finely adjusted after its fabrication in its resonance frequency and accelerating electric-field distribution by adjusting the insertion amount of external tuner blocks composed of from several to several tens blocks and inserted in the acceleration cavity (For example, Patent Document 1 and Patent Document 2).
Patent Document 1: Japanese Patent Application Laid-open No. 2007-157400 (FIG. 1)
Patent Document 2: Japanese Patent No. 4194105 (FIGS. 1-3)
An amount of high frequency power to be supplied to the acceleration cavity for generating the accelerating electric field, is determined by power consumption in the acceleration cavity and an amount of beam loading. The power consumption in the acceleration cavity is categorized into that due to a surface resistance and that due to a contact resistance, in the acceleration cavity. Generally, assuming that the power consumption due to the surface resistance is a value of 1, the power consumption due to the surface resistance and the contact resistance in combination is represented as 100/80 to 100/60. Accordingly, an increase of the number of devices in the acceleration cavity that produce a contact resistance, causes an increase in power consumption in the acceleration cavity, resulting in increase of a capacity of the high frequency power source that generates high frequency power to be supplied to the acceleration cavity. Thus, in the case of using a drift tube linear accelerator as the injector of a synchrotron, if a large number of external tuners are disposed as in the conventional art according to the necessity to highly accurately adjust the resonance frequency and the accelerating electric-field distribution, the power consumption due to the surface resistance and the contact resistance in combination is more increased, resulting in a problem that the capacity of the high frequency power source becomes increased.